Means for monitoring the safety of a remote electrical ground connection

ABSTRACT

The invention contemplates safety monitoring of the condition of a remote electrical ground with reference to a given base end, as in application to elongate flexible trailing cable of the character used to supply large quantities of electrical power from a base or source-connection end to a movable load-connection end, which may be as much as several miles remote from the base end. Aside from power conductors, the trailing cable may have a ground conductor and one or more pilot conductors, providing a fail-safe circuit through pilot and ground conductors, whereby power is automatically disconnected at the source-connection end, should there be an interruption in the fail-safe circuit. The invention provides additional circuit elements utilizing the existing ground conductor and other of said conductors for monitoring the true instantaneous resistance between the sourceconnection and load-connection ends of the ground and, in a preferred form, for automatically interrupting the fail-safe circuit when the monitored ground-conductor resistance is found to reach a predetermined unsafe level, as for example 4 ohms or less.

United States Patent Scarpino 1 May 27, 1975 1 1 MEANS FOR MONITORING THE SAFETY OF A REMOTE ELECTRICAL GROUND CONNECTION [75] Inventor: Leopoldo Scarpino, lndianapolis,

Ind.

[73] Assignee: AMAX Inc., New York, NY.

[22] Filed: May 9, 1973 [2]] Appl. No.: 358,468

[52] U.S. Cl 317/18 C; 317/45; 324/51 [51] Int. Cl. H02h 3/16 [58] Field ofSearch 317/18 R, 18 A. 18 C, 45; 324/51 [56] References Cited UNITED STATES PATENTS 3,728582 4/1973 Agnew 317/13 C 3 746929 7/1973 Kotheimer 317/18 A Primary Examiner-lames D. Trammell Attorney, Agent, or Firm-Kasper T. Serijan; Roy C. Hopgood [57] ABSTRACT The invention contemplates safety monitoring of the condition of a remote electrical ground with reference to a given base end, as in application to elongate flexible trailing cable of the character used to supply large quantities of electrical power from a base or sourceconnection end to a movable load-connection end, which may be as much as several miles remote from the base end. Aside from power conductors, the trailing cable may have a ground conductor and one or more pilot conductors, providing a fail safe circuit through pilot and ground conductors. whereby power is automatically disconnected at the sourceconnection end, should there be an interruption in the fail-safe circuit. The invention provides additional circuit elements utilizing the existing ground conductor and other of said conductors for monitoring the true instantaneous resistance between the sourceconnection and load-connection ends of the ground and, in a preferred form, for automatically interrupting the fail-safe circuit when the monitored groundconductor resistance is found to reach a predetermined unsafe level, as for example 4 ohms or less.

21 Claims, 6 Drawing Figures MID- CNA/EL'fM/V END H I I l i e g 54K 0 new 22 2 Ram/0 dam/awe 11 r 25 25 l U4 zany L l.l

M raw 0. (mac r6 5490 rs Zack/1v: kooks- PATENTED MY 2 7 1915 SHEET wi y MEANS FOR MONITORING THE SAFETY OF A REMOTE ELECTRICAL GROUND CONNECTION This invention relates to safety circuitry for use with trailing cables or the like wherein there must be assurance that remote apparatus cannot develop lethal electrical ground potentials with respect to earth potential.

ln the supply of electrical power to heavy and distant moving industrial machines such as strip-mining shovels, a single multiple-conductor trailing cable provides flexible connection of the remote machine to its base or sourceconnection end. Very often a major source connection will supply a plurality of trailing cables, serving different-size loads at a variety of remote and moving destinations, and using a variety of powerconductor voltages as appropriate for each load and its distance from the base; in other situations. extension, tapping or branch trailing-cable connections of various capacities are made to a single major trailing cable, at locations along the length of the major cable, and with extension or branch-cable capacities and lengths appropriate to the several different loads served thereby. In general, it can be said that whatever the capacity of a major or branch cable, it has (a) its own sourceconnection end at which circuit breakers serve exclusively to protect electric power supplied to the particular cable (neutral-to-ground resistance being selected for an ohmic value which will limit current flow due to a power fault to ground so as not to exceed 25 amperes), (b) a load-connection end, which may be a traveling machine, or a step-down transformer for supply of one of more further branch cables, (r') a ground con ductor which is relied upon to maintain a prescribed degree of electric integrity of all cable and load grounds with respect to ground potential at the sourceconnection end, and (d) a pilot conductor which is re lied upon to maintain current in a fail-safe circuit through the associated ground conductor, so that the applicable circuit breakers will trip in the event of current failure in the fail-safe circuit (e) the power conductors, ground conductor and pilot conductor all forming mutually insulated elements of the particular trailing cable. As used in this specification, the expression trailing cable" will be understood to apply to a given length, served from a source-connection end, and serving a loadconnection end, via multiple-conductor flexible cable, such as that just described, regardless of whether the particular length is a major cable (e.g., three-phase power distribution at l5 kilovolts) or a branch cable (e.g., at 6,900 or 440 volts).

The schemes which have to date been devised to as- Sure against development of excessive ground potentials (to earth) in trailing-cable systems of the character indicated are many and are in general not practical. For example, a series-connected" variety uses a dc source at the source-connection end of the pilot-line arm of the fail-safe circuit, the latter being completed by connection of the ends of said arm to the corresponding ends of the ground conductor. In such a system, the fail-safe relay at the source-connection end of the pilot line is set to drop out (i.e., to trip the associated breakers) at a preselected minimum level of fail safe current. on the assumption that such level is indicative of excessive resistance in the ground conductor; the difficulty with such a system is that it does not and Cannot assuredly operate on the resistance of the ground wire, due to its vulnerability to resistancereducing effects of shunt paths. Also, a "shuntconnected" variety places the coil of the fail-safe relay in shunt across the d-c source and a suitable resistor. to monitor for current build-up beyond a predetermined level, as in the case of a breakage in the ground con ductor. Neither of these kinds of solution is adequate to the task of always tripping the breakers should the ground wire itself increase in resistance above a predetermined level, short of the minimum earth resistance between the source-connection and load-connection ends of the cable.

It is, accordingly, an object of the invention to provide an improved safety mechanism of the character indicated.

Another object is to provide such an improved mechanism without requiring any change in cable construction.

A specific object is to provide such an improved mechanism which is sensitive essentially only to groundconductor resistance, and at a predetermined level less than the minimum local earth resistance and greater than the good-condition or equal-to-new resistance of the ground conductor Another specific object is to provide such an improved mechanism which will assuredly operate to trip the associated breakers when detected ground conductor resistance is at least no greater than four ohms.

A further specific object is to provide such a system wherein that resistance value can be selected, upon the detection of which value as the resistance of the ground conductor, the associated breakers will automatically shut down the supply of power to the trailing cable.

A general object is to achieve the foregoing objects with basically simple and reliable circuitry, requiring little or no adjustment, monitoring or maintenance; mechanism that is relatively fool-proof and involves the use of available, non-critical components, and which at the same time lends itself to the protection of a variety of different sizes, lengths and capacities of trailing cable.

Other objects and various further features of novelty and invention will be pointed out or will occur to those skilled in the art from a reading of the following specification, in conjunction with the accompanying drawings. in said drawings, which show illustrative Preferred forms of the invention.

FIG. 1 is a diagram schematically showing one embodiment of the invention, in application to a trailing cable and its power-supply and load connections;

FIGS. 2 to 4 are simplified similar diagrams to show further embodiments; and

FIGS. 1A and 2A are very much simplified diagrams to illustrate an aspect of the basic organization of FIGS. 1 and 2 respectively.

The system of FIG. 1 is seen to employ a liveconductor trailing cable which will be understood to extend flexibly between a source-connection end and a load-connection end, all as generally indicated by legend in the drawing. The system is illustrative of the principle of resistance-monitoring in the ground con ductor, utilizing constant current in a series-connected fail-safe circuit which includes the ground conductor. and observing voltage drop between ends of the ground conductor.

Specifically, the ground-connection end is shown to include a thrce-phasc power-source connection comprising a transformer having A-connected primary windings and Y-connected secondary windings. the secondary neutral" or "center" having a grounding resistance 11 in its connection to the point 12 of system-grounding to earth. Three-phase power is supplied through line circuit breakers 13 to a delta-connected load 14, via three power conductors 15-16-17. The trailing-cable length between the source-connection and load-connection ends will be understood to be flexible, adapted to lie on the surface ofthe terrain, and to extend various distances, for example up to 5 or more miles, the conductor size and transmission voltage being as appropriate for the load and distance requirements imposed on the cable shown. The loadconnection end will be understood to comprise a metal housing or machine frame 18, indicated by dashed out line and having a terminal 19 by which the ground conductor 20 of the cable is directly clamped to the system-ground terminal 12; the indeterminate relation of the load-frame ground to local-earth ground is suggestcd by dashed-line symbolism at 21. The circuit of a series-connected fail-safe relay 22 is shown to include ground conductor 20 and a pilot-arm. The pilot arm is connected to ground conductor 20 at the systemground terminal 12 and additionally includes the coil of relay 22, a d-c source 23, means 24 for the regulation ofd-c current to a predetermined level (as suggested by an arrow symbol], a local ammeter 25, a relay 26 with normally closed contacts which are openable to interrupt current flow in the fail-safe circuit, and local test elements comprising normally closed contacts 27 bridging a test resistor 28. The completed circuit through the thus-far described parts bears a functional resemblance to series-connected systems of the prior art.

ln accordance with the invention, means are provided at one or both of the connection ends of the trailing cable whereby ground-conductor resistance is monitored by a direct measurement between the groundconnection terminals, i.e., with substantially the same effectiveness as if one could take a familiar volt-ohm meter and apply its respective leads to terminals 12-19, in spite of the great physical distance between these terminals.

In the form of FIG. 1, wherein the regulating means 24 (which may be a variable resistor of ohmic value substantially exceeding the ohmic resistance of the ground wire) serves to maintain substantially constant current in the fail-safe circuit, therefore, voltage measured between terminals 12-19 is directly indicative of ground-conductor resistance. Such a voltageresponsive means is schematically indicated at 30, at the load-connection end; as shown, one of the voltagesensing lea-ads" 31 of means is connected to the machine frame. ie, terminal 19, and the other lead 32 makes use of the dc capabilities of neutral-connections to the respective ends of the power conductors 15-]- 0-17, without in any way impairing or being adversely affected by the relatively great quantities of threephase power being delivered to and consumed by load 14. If the load 14 were Y-connected, direct connection of the voltage-sensing lead 32 could be made thereto, preferably through a suitably large protective grounding resistor; but in view of the delta-connection of load 14, I employ a network 33 of like resistors 34-34'34" connected to the respective power conductors, and establishing a local -neutral" at their common connection to lead 32. Also, preferably a by-pass capacitor 35 bridges the voltage-responsive means 30, to assure a-c elimination from the desired d-c voltage response,

Thus, whether the load is delta or Y-connected, lead 32 5 has a local d-c or neutral connection to the power conductors, and all three power conductors 15-16-17 provide the means of effectively extending lead 32 to the system-ground terminal 12. The net effect is to produce a circuit having the general character depicted in FIG. 1A, wherein source 23 circulates known current through the ground conductor, designated R1, and wherein the means 30 for observing voltage deep across R includes resistor 11 and the remaining power-linc resistance, collectively designated R meaning the effective resistance of the parallel power-line elements 34-17, 34'-16, and 34"15.

Voltage-responsive means 30 may be of the very high impedance variety, such as a vacuum-tube voltmeter, so that it does not rely upon drawing d-c or other current via the lengthy lead connection to terminal 12', moreover, as will be seen from the simplified diagram of FIG. 1A, the resistance in series with the voltageresponsivc means 30 (ie, at 11 and R is so many orders of magnitude greater than the ground-conductor resistance R that the impedance of the voltage responsive means 30 cannot be considered to be in any sense critical. Also, as a straight voltmeter, the means 30 will be further understood, with a scale calibrated in ohms, to be directly and correctly readable in terms of true resistance between terminals 12-19.

Still further symbolism appears in the drawing, to illustrate automatic functioning of the described circuit. The voltage-responsive means 30 is indicated by legend to include a thresholdoperating feature, whereby upon response to a detected dc voltage of magnitude predetermined as by manual selection and adjustment at a knob 36, an output signal is produced in a control con nection 37 to the coil of relay 26, such output signal being operative to open the normally closed contacts of relay 26 and thus to interrupt the normal flow of cur rent in the fail-safe circuit. Such interruption will be recognized at relay 22, since its picked-up condition can no longer be maintained, and all circuit breakers 13 will trip, to de-energize the power conductors and the load.

The drawing also shows, at 27-28, means for testing the operability of the automatic circuitry which has been described. Normally, the closed contacts 27 will enable the voltageresponsive means 30 to make a true resistance measurement of ground conductor 20. However, to make sure that the automatic circuitry is correctly operative, a push button need only be pressed to open contacts 27, thus causing means 30 to additionally sense the voltage drop across resistor 28, it being understood that resistor 28 is selected of such magnitude, for the constant current regulated at 24, to assure appearance of an seemingly "dangerous" groundconductor resistance, cg, 4 ohms or more. In the circumstances, means 30 has no choice but to produce the relay-operating output in line 37, whereby breakers 13 are caused to trip as part of a test procedure. It will also be understood that the test elements 27-28 can be anywhere in series with the ground conductor and in the 6g fail-safe dircuit, as for example at the sourceconnection end of the system, as long as the test elements 27-28 are embraced by the voltage response of means 30.

In normal operation, d-c current flows continuously in the fail-safe circuit comprising the pilot and ground conductors, and the closed contacts at 26-27, thus maintaining a hold-in supply of current to the coil of fail-safe relay 22, so that breakers 13 continuously enable full power to pass to the load 14. In such normal operation, the d-c current remains substantially constant, so that voltage drop between terminals 12-19 is a true indication of ground-conductor resistance, under the constant surveilance of the voltage-responsive threshold means 30. Periodically, operation can be checked out by opening the test contacts 27, causing threshold detection at 30, with resultant opening of relay contacts at 26, drop-out of the fail-safe relay, and tripping of breakers 13. The same result will also ensue if means 30 should detect the threshold without operation of test contacts 27, meaning of course that the detected increase in ground conductor resistance is a true cause for automatic alarm, here noted by trip of the breakers 13.

Stated in more practical illustrative terms, for grounding resistor 11 selected to limit a ground-topower conductor fault to 25 amperes, and for a 0.5- ampere fail-safe current selected at 24, and for means 36 set to trip at 2 volts (meaning a limiting groundconductor resistance of 4 ohms), no short circuit, as from a power conductor to the ground conductor, can ever develop a ground-conductor voltage greater than 100 volts (25 amperes. times 4 ohms); thus, the greatest personnel exposure to ground-conductor voltage will be held to a short transient of no more than 100 volts. It will be understood that, with changed settings, the described system has the inherent capacity to protect as to an even lower limiting condition, the presently illustrative values being supplied merely because they realize the currently stated Bureau of Mines safety objective, which to my knowledge has not hitherto been reliably realized by any other ground-protection system,

The circuit arrangement of FIG. 2 bears certain resemblance to that of FIG. 1 and, therefore, corresponding reference numerals are used where appropriate. The principal difference resides in the technique for monitoring the ground conductor terminals 12-19 as to a predetermined resistance threshold.

In FIG. 2, polarity-sensitive voltage-responsive means 40 is employed to monitor voltage across opposed terminal corners 41-42 of a Wheatstone bridge, wherein the ground conductor 20 is connected at 42 to a variable resistor 43 to define one of two conjugate arms between opposed bridge-excitation terminal corners 44-12. The other conjugate arm between the excitation terminal corners 44-12 comprises the ground resistor 11 plus the effective resistance of network 33, all as connected at 41 to a further resistor 45. Voltage excitation for bridge terminals 44-12 may be developed from a dc source 23 at the source-connection end of the pilot arm to terminal 44; however, in the preferred form shown, source 23 is at the load-connection end of the pilot arm. Thus, fail-safe relay 22 is normally heldin" through flow of continuous current in the fail-safe circuit, from source 23 to terminal 44, resistor 43, testing contacts 27, terminal 19, and back via ground conductor 20 to the system ground 12 and its connection 38 to source 23. The simplified circuit diagram of FIG. 2A utilizes the same simplifying techniques as employed in H0. 1A, but in application to FIG. 2', thus,

the effective power-line resistance R and the resistance at 11 determine the high-resistance bridge arm between points 41-12, and the ground-conductor resistance R is the sole element in the unknown arm of the bridge, between points 12-19 (42).

Preferably, resistor 45 is selected for a resistance value which substantially matches the resistance between points 41-12, namely the combined effective resistance of network 33 and grounding resistor 11. Thus, if resistors 34-34'34" are in the order of 10 megohms and resistor 11 is negligible by comparison, the effective resistance between points 12 and 41 will be substantially 3.3 megohms, and resistor 45 is preferably selected at 3.3 megohms. The balanced condition of the bridge will therefore exist when the ground-conductor resistance (terminal 12 to terminal 19-42) equals the value set at resistor 43, and this value at 43 should be set to match the desired limiting resistance in the ground conductor 20 (e.g., 4 ohms to meet Bureau of Mines requirements, or preferably something less, such as 3 ohms, to exceed the Bureau of Mines requirement). Thus, in a normal operating situation wherein the actual ground-conductor resistance is in the order of l or 2 ohms, or is otherwise less than the limiting magnitude preselected at 43, the polarity-sensitive voltage-responsive means will indicate or otherwise function in accordance with a first polarity status, for the d-c potential of terminal 42 with respect to terminal 41; however, should the ground-conductor resistance sufficiently increase (for example, through the resistance-degrading effect of having run bulldozer cleats over the trailing cable) to equal or exceed the resistance threshold preselected at 43, the bridge will pass through a balanced conditions, with reversal of the polarity of the potential across terminals 41-42. The means 40 will be understood to produce a resulting out put in line 46 to relay 26, causing an opening of the contacts of relay 26 upon such polarity reversal, and as previously indicated, such an opening in the pilot arm of the fail-safe circuit will produce drop-out of relay 22 and a trip of the breakers 13.

Also as previously described, and for a proper selection of test resistance at 28, a test opening of contacts 27 will create for means 40 the appearance of a dangerous resistance development in conductor 20, causing relays 26-22 to perform a power shutdown in the power conductors 14-15-16.

The embodiment of FIG. 3 illustrates another Wheatstone bridge arrangement, wherein previously described corresponding parts are given the same reference numerals. The difference for the case of FIG, 3, is that the trailing cable has two pilot conductors, labeled Pilot A and Pilot B, respectively, and no use of the power conductors 14-15-16 is made for operation of the bridge.

In FIG. 3, the conjugate arm comprising ground conductor 20 and variable resistor 43 (interconnected at 42) may be as described for FIG, 2. However, the other conjugate arm comprises an additional resistor 47 and the Pilot-B conductor, connected at 41 to resistor 45. Bridge excitation from source 23 is applied to terminals 12-44, and resistors 45-47 are preferably selected to be substantially matched and of relatively large value, so that the fail-safe current will still circulate primarily by way of the ground conductor 20 and the Pilot-A arm, which includes relay 22, the normally closed contacts of relay 26, and the normally closed testing contacts 27. Operation is as described for FIG. 2.

Although H0. 4 is virtually identical to HG. 3. it will be rccogni/eil as illustrating a principle that i applicable to all other disclosed forms of the imention.

ln Ht]. 4. the load-connection end of the ground conductor is brought to the local machinc-ground ter minal I) by way of normally closed contacts 50, which may be push-button operated. as suggested at 51. I bus. at test procedure involving opening of contacts 50 will present the voltage-responsive means 40 with a resistance which represents the instantaneous true earth rcsistatice lvia 21! and all possible paths to the system ground 12; preferably, relay 26 is temporarily disabled during the measurement so as to avoid unwitting shutdown of power in conductors 1445-16. and I provide normally open contacts 52 across the normally closed contacts of relay 26, contacts 52 being interlocked with contacts 50 (as suggested by dashedline mechanical interconnection l to assure closure at 52 whenever contacts 50 are pressed open for an earth-ground resis tance measurement. When contacts 50 are thus actu atcd, resistor 43 may he adjusted to achieve bridge balance lzero-indicated volts at 40), whereupon an indicating scale for the adjustment at 43 will be direct reading for the thewexisting effective earth-ground resistance between the source'connection and loadconnection ends of the system.

The described embodiments of the invention will be seen to have achieved all stated objects with structure of elemental simplicity and using non-critical components. In particular, the invention is realizable with minimum modification of existing trailing-cable installations in the field, and in all cases it is possible not only to monitor for the onset of a predetermined unsafe level of groundconductor resistance, but also to test the monitoring efficiency as well as to actually measure the then-existing earthground resistance back to the system ground 12.

According to current Bureau of Mines requirements, a grounding resistor such as the resistor 11 for the sup plydine neutral at the source-connection end must be of such ohmic value as to limit the voltage drop in the grounding circuit external to the resistor to no more than llltl volts under fault conditions and also to be such as to limit the ground fault current to no more than 25 amperes. thus establishing the above noted -l-ohm limitation for ground'conductor resistance. It is believed that the present invention for the first time provides a means whereby this important ohmic value can be continuously monitored. And such monitoring is achieved while also meeting Bureau of Mines re quircmcnts as to operation of the fail-safe groundcheck circuit, meaning that the breakers 13 will be tripped in either the pilot conductor or the ground conductor should open tie, break l. Moreover. the nature and connection of the threshold responsive means (30, 40) are such as to be inherently relatively blind or insensitive to unbalanced load transients that may be induced in any part of the fail safe circuit, so that the intention is operative where other systems cannot be relied upon. and using cable of virtually any sectional eometry. including asymmetrical cable sections.

p fhe iinention provides still further desirable features; in that:

u. The invention is operative with any length of cable.

with only minor adjustment,

(ill

b. As long as the source 23 of fail-safe circuit current is at the remote end of the system. the breakers 13 will trip in the event of a short between pilot and ground conductors;

t' The invention does not require insulating the ground conductor of "lifting" it at any point:

. The invention does not require use of a shunt" or shunts" in the ground or fail-safe circuit;

1. The invention will correctly respond to sense an open ground conductor. even though the cable or cables may be earth-grounded at se\eral points;

1'. The invention operates so to detect resistance and to assure shutdown of power that there will he no hazard in personnel contact with an unsuspected open ground;

, The system is such as inherently not to require any resetting once it has tripped the circuit breakers,

i.e,, upon closing the circuit breakers. any subsequent trip action will be the result of a new threshold evaluation, to actuate relay 26;

It. The basic or modular-unit nature of the invention means that it is compatible with itself in any combination of major and subordinate trailing cables, breakers. couplings, Y or A winding configurations. etc:

i'. The invention does not require frequent resetting of thresholds or critical adjustments, once installed; in fact. adjustments made in the shop, fac tory or otherwise prior to installation may in most cases be correct, without compromise of safety;

j. The bridge-connected threshold systems of H65. 2 to 4 are noncritical as to voltage or current in the tail-safe circuit. in that they all operate on a comparison between voltages at terminals 4l42, the excitation voltage being proportionally divided in like fashion along each of the respective conjugate arms of the bridge; and

k. Groundconductor resistance may be so accurately monitored at a threshold level less than the lowest possible earth resistance that breaker trip through the invention provides a positive indication of trouble in the pilot conductor or in the ground conductor.

While the invention has been described in detail for the preferred forms shown, it will be understood that modifications may be made without departure from the scope of the invention. For example, the invention is not necessarily to be considered limited to threephase power systems but is applicable wherever the necessary control of ground-conductor resistance is to be monitored, in systems which may be other than employed for power distribution. or in which other than threephase power is delivered.

Also, for example, FIGS. 3 and 4 will be understood to schematically indicate elements of a separate article of manufacture for application to the remote end of an existing multiple-conductor cable system having a sys tent ground l2 at the source-connection end. Such article may comprise the indicated interconnected components of virtually the entire Wheatstone bridge. Thus. such a package may contain the interconnected resistors 4547-43 and \oltagc-responsive means 40, in which case the package presents three terminals for external connection in the held. there being included in such package a display of voltage detected by means 40 and a manual adiustment. preferably calibrated in ohms. for the variable resistor 43; the three terminals in such case are (1) a first bridge-excitation terminal, for field connection to the conductor arm (Pilot A) which serially contains the bridge-excitation voltage source, (2) the terminal 53 by which the serially connected fixed resistors 4547 complete one of the conjugate bridge arms upon series-connection in the field to the adjacent end of an independent d-c conducting path to the other bridge-excitation terminal (12), and (3) the terminal 42 by which the variable resistor 43 is serially connected in the field to the load-connection end of the ground circuit to be evaluated by the bridge.

Still further by way of example, FIGS. 3 and 4 will be understood to schematically indicate elements of a further article of manufacture for field-testing of the fidelity of earth ground in a given system ground that is suspected of being less than adequately earth-bound. Such an article will be understood to comprise the elements of the package of the preceding paragraph, and to further include:

a. Elongate flexible conductors (Pilot A and Pilot B), connected to each other at one end (12), wherein the heavy dot at 12 will be understood to suggest means, such as aligator clip, for removable connection to a system ground to be tested, one of these conductors (Pilot A) serially including a voltage source (23) and therefore comprising the means of excitation across bridge terminals 12-44, the other of these conductors (Pilot B) constituting with the resistors 4547 a first conjugate arm of the bridge; and

b. Means symbolized at 19 (or at 42) and comprising for example an aligator clip for removable connection of means 40-43 to another earth-bound system ground, such as a rail of railroad track, the cold-water supply plumbing of a building, etc., thereby completing the other conjugate-arm connections through resistor 43 and the two system grounds and the earth therebetween. In use of such an article, the adjustment at 43 to the point of bridge balance will provide a direct reading of true resistance between the two system grounds and if the ohmic reading is not as low as that to be expected for local earth resistance, it can be known that the suspected system ground is not to be trusted unless and until proper corrective measures have been taken.

What is claimed is:

l. Elongate transmission-line means having a sourceconnection end and a remote load-connection end and comprising power-conductor means, a ground conductor and a pilot conductor; a d-c conducting connection of said power-conductor means to said ground and pilot conductors at the source-connection end; a d-c conducting connection to said power-conductor means at the load-connection end; whereby relatively constant resistance values characterize the d-c conducting paths from the respective load-connection ends of said power-conductor means and of said pilot-conductor means, to said d-c conducting connection at the source-connection end; means for impressing a d-c potential across the full length of said ground conductor, said last-mentioned means including a d-c source in one of said paths and a circuit connection of said one path and said ground conductor at the load-connection ends thereof; and d-c responsive resistance-monitoring means at the load-connection end and including a ref erence connection to the source-connection end of said ground conductor via the other of said paths, said resistance-monitoring means being also connected to the load-connection end of said ground conductor and thus tracking the instantaneous effective resistance between the source-connection and load-connection ends thereof.

2. Transmission-line means according to claim I, in which the path which includes said d-c source is the pilot-conductor path.

3. Transmission-line means according to claim I, in which said resistance-monitoring means includes threshold-operative means for responding to a preselected measured-resistance level, said level being preselected to a maximum tolerable level above the known resistance of the ground conductor in good condition.

4. Transmission-line means according to claim I, in which said resistance-monitoring means includes connected elements ofa bridge circuit having a first excitation terminal at the remote-connection end of said one path and a second excitation terminal at the sourceconnection end of said one path, a first pair of conjugate fixed-resistance arms between said terminals, said fixed-resistance arms being located at said remote connection end and including said other path to establish the connection to said second excitation terminal, a second pair of conjugate arms between said termi nals, one arm of said second pair being the full length of said ground conductor, and the other arm of said second pair comprising a resistance of value preselected to bear substantially the same relation to the normal good-condition resistance value of said ground conductor that said fixed-resistance arms bear to each other, and means for monitoring voltage observed between the point of connection of the first pair of arms to each other and the point of connection ofthe second pair of arms to each other.

5. Transmission-line means according to claim 1, in which said one path includes means for maintaining a substantially constant predetermined dc current flow in said one path, and in which said monitoring means is voltage-responsive.

6. Transmission-line means according to claim 1, in which said power-conductor means comprises multiple conductors connected for the remote transmission of three-phase power, an end of said power-conductor means being Y-connected, the d-c conducting connection to said Y-connected end including a grounding resistance and being connected to the center of the Y.

7. Transmission-line means according to claim I, in which said power-conductor means comprises multiple conductors connected for the remote transmission of three-phase power, an end of said power-conductor means being A-connected, and the d-c conducting connection thereto including separate highresistance arms connected at one end to the respective power conduc tors and interconnected at their other ends to define a neutral point, said d-c conducting connection being to said neutral point.

8. Transmission-line means having a sourceconnection end and a remote load-connection end and comprising multiple power conductors, a ground conductor and a pilot conductor; an independent circuit comprising a pilot arm in which a d-c source is seriesconnected to said pilot conductor, one end of said arm being connected to the corresponding end of said ground conductor. and means connecting the other ends of said pilot arm and ground conductor, whereby a d-c current will normally flow in said independent circuit; and means including a dc conducting connection to the loadconnection end of said power conductors and a d-c conducting connection to the sourceconnection end of said ground conductor for sensing the d-c potential end-to-end across the length of said ground conductor.

9. Transmission-line means according to claim 8, in which the pilobconductor part of said independent circuit includes means for regulating current to a substantially constant flow. whereby said sensed d-c potential is directly responsive to the instantaneous value of re sistance between the source-connection and load- Connection ends of said ground conductor.

10. Transmission-line means according to claim 8, in which said pilot arm and therefore said d-c source is connected across opposed corners of a resistancebridge circuit in which said ground conductor and said sensing means form operative component parts, said bridge circuit further including conjugate fixedresistance arms one of which includes said power conductors and the d-c conducting connections thereto.

1]. Transmission-line means having a sourceconnection end and a remote load-connection end and comprising a ground conductor, a pilot conductor and multiple further conductors including power conductors; circuitbreaker means for said power conductors at the source-connection end; a d-e conducting connection ofat least one of said further conductors to said ground conductor at the source-connection end; a dc conducting connection for said one further conductor at the loadconnection end; an independent circuit comprising a pilot arm in which a d-c source is seriesconnected to said pilot conductor, one end of said arm being connected to the corresponding end of said ground conductor and means connecting the other ends of said pilot arm and ground conductor; means including a d-c conducting connection to the loadconnection end of said one further conductor for sens ing the dc potential from end to end of said ground conductor; and circuit-breaker trip means operatively connected to said sensing means and responsive to trip said circuit-breaker means upon the sensing ofa predetermined level of said d-c potential.

12. Transmission-line means having a sourceconnection end and a remote load-connection end and comprising multiple power conductors, a ground conductor and a pilot conductor; circuit-breaker means for said power conductors at the source-connection end; a d-c conducting connection of said power conductors to said ground conductor at the source-connection end; an independent circuit comprising a pilot arm in which a d-c source is series-connected to said pilot conductor, one end of said arm being connected to the corresponding end of said ground conductor, and means connecting the other ends of said pilot arm and ground conductor; means in said pilot arm for establishing a predetemined d-c current level in said circuit; means at the remote end of said transmissiondine means for sensing the d-c potential on at least one of said power conductors with respect to the remote end of said ground conductor; and circuitbreaker trip means op erative upon detection by said last-defined means of a predetermined dc voltage level.

13. Elongate flexible grounding-line means having an electrically conductive reference-potential end and a remote electrically conductive end to be safely groundreferenced to said reference-potential end and com prising multiple conductors including a ground conductor and plural further conductors; an independent safety-monitoring circuit comprising a pilot arm in which a voltage source is seriesconnected to one of said further conductors, one end of said arm being connected to the corresponding end of said ground conductor and means connecting the other ends of said pilot arm and ground conductor; means connecting the reference-potential ends of another of said further conductors and of said ground conductor; means in the pilot arm of said circuit for establishing a predetermined current level in said circuit; means at the remote end of said grounding-line means for sensing the potential on said another conductor with respect to the remote end of said ground conductor; and safety-threshold means operative upon detection by said last-defined means of a predetermined voltage level.

14. Elongate flexible grounding-line means having an electrically conductive reference-potential end and a remote electrically conductive end to be safely groundreferenced to said reference-potential end and comprising multiple conductors including a ground conductor and plural further conductors; an independent safety-monitoring circuit comprising a pilot arm in which a voltage source is series-connected to one of said further conductors, one end of said arm being connected to the corresponding end of said ground conductor. and means connecting the other ends of said pilot arm and ground conductor; means connecting the reference-potential ends of another of said further conductors and of said ground conductor; means including a connection to the remote end of said another further conductor and a connection to the remote end of said ground conductor for sensing the resistance between ends of said ground conductor; and safety-threshold means operative upon detection by said last defined means of a predetermined resistance level.

15. Elongate flexible groundingline means having an electrically conductive reference-potential end and a remote electrically conductive end to be safely groundreferenced to said reference-potential end and comprising multiple conductors including a ground conductor and a pilot conductor having a conductive connection to each other at one end, and resistance-bridge elements connected to the other ends of said ground and pilot conductors; said bridge elements comprising con nected conjugate fixed-resistance members between opposed bridge-excitation points, a voltage source connecting one of said points to to said other end of said pilot conductor. a monitoring resistance selected for value substantially corresponding to the desired safetylimit value of said ground conductor and in substantially the same proportional relationship therewith as the values of said fixed resistance members bear to each other. said monitoring resistance connecting the said other end of said ground conductor to said one point. thereby defining second conjugate members connected between said points and voltage-responsive means interconnecting said conjugate arms at the respective member-to-member connections thereof.

16. Transmissiondinc means having a sourceconnection end and a remote load-connection end and comprising multiple power conductors. a ground conductor and a pilot conductor. said ground conductor and pilot conductor being connected to each other at one of said ends; circuit-breaker means for said power conductors at the source-connection end; resistance bridge elements connected to said ground and pilot conductors at said other end, said bridge elements comprising connected conjugate fixed-resistance members between opposed bridge-excitation points, a voltage source connecting one of said points to said other end of said pilot conductor. a monitoring resistance selected for value substantially corresponding to the desired safety-limited value of said ground conductor and in substantially the same proportional relationship therewith as the values of said fixed-resistance members bear to each other, said monitoring resistance con necting said other end of said ground conductor to said one point, thereby defining second conjugate members connected between said points, and voltage-responsive means interconnecting said conjugate arms at the re spective member-to-member connections thereof; and circuit-breaker trip means operative upon detection by said last-defined means of a predetermined voltage level 17. Transmission-line means according to claim 14, in which said fixed-resistance members are selected for substantially equal value, and in which said trip means is operative upon detection of a voltage level of polarity opposite to that which is detected when the groundconductor resistance is observed to be less than that of said monitoring resistance.

18. Transmission-line means having a sourceconnection end and a remote load-connection end and comprising a ground conductor and multiple further conductors, including power conductors; a d-c conducting connection of at least one of said multiple conductors to said ground conductor at one of said ends, said at least one conductor including a d-c connection at said other end; circuitbreaker means for said power conductors at the source-connection end; resistancebridge elements connected to said ground conductor and to the d-c connection of said at least one conductor at said other end, said bridge elements comprising connected conjugate fixed-resistance members between opposed bridge-excitation points, a d-c voltage source connecting one of said points to the d-c connection at said other end of said at least one conductor, a monitoring resistance selected for value substantially corresponding to the desired safety limit value of said ground conductor and in substantially the same proportional relationship therewith as the values of said fixedresistance members bear to each other, said monitoring resistance connecting said other end of said ground conductor to said one point, thereby defining second conjugate members connected between said conjugate arms at the respective member-to-member connections thereof; and circuit-breaker trip means operative upon detection by said last-defined means of a predetermined voltage level,

[9. Transmission line means according to claim 16, in which a system ground to earth is provided at said one end of said ground conductor and in which said other end includes a local-grounding frame member, and means including normally closed contacts at said other end and connecting the corresponding end of said ground conductor to said frame member. the ground connection of said bridge elements at said other end being made to said frame member, whereby upon opening said contacts said voltage-responsive means and said monitoring resistance may serve to measure then-existing earth resistance to the system ground via such path or paths as may exist between said frame member and the local electrical relation thereto to earth.

20. Transmission-line means having a sourceconnection end and a remote load-connection end and comprising a ground conductor and multiple further conductors, including power conductors; a first d c conducting connection of at least one of said multiple conductors to said ground conductor at one of said ends, said at least one conductor including a d-c connection at said other end; a second d-c conducting connection of another of said multiple conductors to said ground conductor at said one end; circuit-breaker means for said power conductors at the sourceconnection end; Wheatstone-bridge elements at the other of said ends and utilizing, as one corner of the bridge, the said interconnection of multiple conductors and ground conductor at said one end, so that the ground conductor itself may be one of the arms of a completed bridge; said bridge elements at said other end comprising two serially connected resistance arms of ohmic value much higher than that for which the ground conductor is to be monitored. thereby defining a second bridge corner at the interconnection of said high-resistance arms, a low-resistance arm of ohmic value comparable to that for which the ground conductor is to be monitored, said low resistance arm being connected to the otherwise-free end of one of said high-resistance arms, thereby defining a third bridge corner at interconnection of high and lowresistance arms, and thereby also defining first and second otherwise-available free ends of a high resistance arm and of the low resistance arm respectively; means including said at least one conductor and connecting one of said otherwise-available free ends to said first'defined bridge corner, and means connecting the other of said otherwise-available free ends to said other end of said ground conductor, thereby defining the fourth bridge corner, and thereby establishing the serieslo0p con' nection of all four bridge corners via said arms; a voltage-excitation source diagonally connecting a first two opposite bridge corners, and voltage-responsive means diagonally connecting the remaining two opposite bridge Corners, said source and voltage-responsive means being located at said other end, and said another conductor being the means of completing the requisite one of said diagonal connections to said first-defined corner; and means responsive to attainment of a predetermined response of said voltage-responsive means for tripping said circuit-breaker means 21. Transmissiomline means according to claim 20, in which the high-resistance arms are of substantially matched ohmic value, and in which the ohmic value of the low-resistance arm is to a predetermined safetymonitoring level of ground-conductor resistance, said voltage-responsive means being polarity sensitive and operative to actuate said last-defined means upon detection of a polarity reversal of voltage, 

1. Elongate transmission-line means having a source-connection end and a remote load-connection end and comprising powerconductor means, a ground conductor and a pilot conductor; a d-c conducting connection of said power-conductor means to said ground and pilot conductors at the source-connection end; a d-c conducting connection to said power-conductor means at the loadconnection end; whereby relatively constant resistance values characterize the d-c conducting paths from the respective loadconnection ends of said power-conductor means and of said pilotconductor means, to said d-c conducting connection at the sourceconnection end; means for impressing a d-c potential across the full length of said ground conductor, said last-mentioned means including a d-c source in one of said paths and a circuit connection of said one path and said ground conductor at the load-connection ends thereof; and d-c responsive resistancemonitoring means at the load-connection end and including a reference connection to the source-connection end of said ground conductor via the other of said paths, said resistance-monitoring means being also connected to the load-connection end of said ground conductor and thus tracking the instantaneous effective resistance between the source-connection and load-connection ends thereof.
 2. Transmission-line means according to claim 1, in which the path which includes said d-c source is the pilot-conductor path.
 3. Transmission-line means according to claim 1, in which said resistance-monitoring means includes threshold-operative means for responding to a preselected measured-resistance level, said level being preselected to a maximum tolerable level above the known resistance of the ground conductor in good condition.
 4. Transmission-line means according to claim 1, in which said resistance-monitoring means includes connected elements of a bridge circuit having a first excitation terminal at the remote-connection end of said one path and a second excitation terminal at the source-connection end of said one path, a first pair of conjugate fixed-resistance arms between said terminals, said fixed-resistance arms being located at said remote-connection end and including said other path to establish the connection to said second excitation terminal, a second pair of conjugate arms between said terminals, one arm of said second pair being the full length of said ground conductor, and the other arm of said second pair comprising a resistance of value preselected to bear substantially the same relation to the normal good-condition resistance value of said ground conductor that said fixed-resistance arms bear to each other, and means for monitoring voltage observed between the point of connection of the first pair of arms to each other and the point of connection of the second pair of arms to each other.
 5. Transmission-line means according to claim 1, in which said one path includes means for maintaining a substantially constant predetermined d-c current flow in said one path, and in which said monitoring means is voltage-responsive.
 6. Transmission-line means according to claim 1, in which said power-conductor means comprises multiple conductors connected for the remote transmission of three-phase power, an end of said power-conductor means being Y-connected, the d-c conducting connection to said Y-connected end including a grounding resistance and being connected to the center of the Y.
 7. Transmission-line means according to claim 1, in which said power-conductor means comprises multiple conductors connected for the remote transmission of three-phase power, an end of said power-conductor means being Delta -connected, and the d-c conducting connection thereto including separate high-resistance arms connected at one end to the respective power conductors and interconnected at their other ends to define a neutral point, said d-c conducting connection being to said neutral point.
 8. Transmission-line means hAving a source-connection end and a remote load-connection end and comprising multiple power conductors, a ground conductor and a pilot conductor; an independent circuit comprising a pilot arm in which a d-c source is series-connected to said pilot conductor, one end of said arm being connected to the corresponding end of said ground conductor, and means connecting the other ends of said pilot arm and ground conductor, whereby a d-c current will normally flow in said independent circuit; and means including a d-c conducting connection to the load-connection end of said power conductors and a d-c conducting connection to the source-connection end of said ground conductor for sensing the d-c potential end-to-end across the length of said ground conductor.
 9. Transmission-line means according to claim 8, in which the pilot-conductor part of said independent circuit includes means for regulating current to a substantially constant flow, whereby said sensed d-c potential is directly responsive to the instantaneous value of resistance between the source-connection and load-connection ends of said ground conductor.
 10. Transmission-line means according to claim 8, in which said pilot arm and therefore said d-c source is connected across opposed corners of a resistance-bridge circuit in which said ground conductor and said sensing means form operative component parts, said bridge circuit further including conjugate fixed-resistance arms one of which includes said power conductors and the d-c conducting connections thereto.
 11. Transmission-line means having a source-connection end and a remote load-connection end and comprising a ground conductor, a pilot conductor and multiple further conductors including power conductors; circuit-breaker means for said power conductors at the source-connection end; a d-c conducting connection of at least one of said further conductors to said ground conductor at the source-connection end; a d-c conducting connection for said one further conductor at the load-connection end; an independent circuit comprising a pilot arm in which a d-c source is series-connected to said pilot conductor, one end of said arm being connected to the corresponding end of said ground conductor, and means connecting the other ends of said pilot arm and ground conductor; means including a d-c conducting connection to the load-connection end of said one further conductor for sensing the d-c potential from end to end of said ground conductor; and circuit-breaker trip means operatively connected to said sensing means and responsive to trip said circuit-breaker means upon the sensing of a predetermined level of said d-c potential.
 12. Transmission-line means having a source-connection end and a remote load-connection end and comprising multiple power conductors, a ground conductor and a pilot conductor; circuit-breaker means for said power conductors at the source-connection end; a d-c conducting connection of said power conductors to said ground conductor at the source-connection end; an independent circuit comprising a pilot arm in which a d-c source is series-connected to said pilot conductor, one end of said arm being connected to the corresponding end of said ground conductor, and means connecting the other ends of said pilot arm and ground conductor; means in said pilot arm for establishing a predetemined d-c current level in said circuit; means at the remote end of said transmission-line means for sensing the d-c potential on at least one of said power conductors with respect to the remote end of said ground conductor; and circuit-breaker trip means operative upon detection by said last-defined means of a predetermined d-c voltage level.
 13. Elongate flexible grounding-line means having an electrically conductive reference-potential end and a remote electrically conductive end to be safely ground-referenced to said reference-potential end and comprising multiple conductors including a ground conductor and plural further conductors; an independent saFety-monitoring circuit comprising a pilot arm in which a voltage source is series-connected to one of said further conductors, one end of said arm being connected to the corresponding end of said ground conductor, and means connecting the other ends of said pilot arm and ground conductor; means connecting the reference-potential ends of another of said further conductors and of said ground conductor; means in the pilot arm of said circuit for establishing a predetermined current level in said circuit; means at the remote end of said grounding-line means for sensing the potential on said another conductor with respect to the remote end of said ground conductor; and safety-threshold means operative upon detection by said last-defined means of a predetermined voltage level.
 14. Elongate flexible grounding-line means having an electrically conductive reference-potential end and a remote electrically conductive end to be safely ground-referenced to said reference-potential end and comprising multiple conductors including a ground conductor and plural further conductors; an independent safety-monitoring circuit comprising a pilot arm in which a voltage source is series-connected to one of said further conductors, one end of said arm being connected to the corresponding end of said ground conductor, and means connecting the other ends of said pilot arm and ground conductor; means connecting the reference-potential ends of another of said further conductors and of said ground conductor; means including a connection to the remote end of said another further conductor and a connection to the remote end of said ground conductor for sensing the resistance between ends of said ground conductor; and safety-threshold means operative upon detection by said last-defined means of a predetermined resistance level.
 15. Elongate flexible grounding-line means having an electrically conductive reference-potential end and a remote electrically conductive end to be safely ground-referenced to said reference-potential end and comprising multiple conductors including a ground conductor and a pilot conductor having a conductive connection to each other at one end, and resistance-bridge elements connected to the other ends of said ground and pilot conductors; said bridge elements comprising connected conjugate fixed-resistance members between opposed bridge-excitation points, a voltage source connecting one of said points to to said other end of said pilot conductor, a monitoring resistance selected for value substantially corresponding to the desired safety-limit value of said ground conductor and in substantially the same proportional relationship therewith as the values of said fixed-resistance members bear to each other, said monitoring resistance connecting the said other end of said ground conductor to said one point, thereby defining second conjugate members connected between said points, and voltage-responsive means interconnecting said conjugate arms at the respective member-to-member connections thereof.
 16. Transmission-line means having a source-connection end and a remote load-connection end and comprising multiple power conductors, a ground conductor and a pilot conductor, said ground conductor and pilot conductor being connected to each other at one of said ends; circuit-breaker means for said power conductors at the source-connection end; resistance-bridge elements connected to said ground and pilot conductors at said other end, said bridge elements comprising connected conjugate fixed-resistance members between opposed bridge-excitation points, a voltage source connecting one of said points to said other end of said pilot conductor, a monitoring resistance selected for value substantially corresponding to the desired safety-limited value of said ground conductor and in substantially the same proportional relationship therewith as the values of said fixed-resistance members bear to each other, said monitoring resistance connecting said other end of said ground conductor to said one poInt, thereby defining second conjugate members connected between said points, and voltage-responsive means interconnecting said conjugate arms at the respective member-to-member connections thereof; and circuit-breaker trip means operative upon detection by said last-defined means of a predetermined voltage level.
 17. Transmission-line means according to claim 14, in which said fixed-resistance members are selected for substantially equal value, and in which said trip means is operative upon detection of a voltage level of polarity opposite to that which is detected when the ground-conductor resistance is observed to be less than that of said monitoring resistance.
 18. Transmission-line means having a source-connection end and a remote load-connection end and comprising a ground conductor and multiple further conductors, including power conductors; a d-c conducting connection of at least one of said multiple conductors to said ground conductor at one of said ends, said at least one conductor including a d-c connection at said other end; circuit-breaker means for said power conductors at the source-connection end; resistance-bridge elements connected to said ground conductor and to the d-c connection of said at least one conductor at said other end, said bridge elements comprising connected conjugate fixed-resistance members between opposed bridge-excitation points, a d-c voltage source connecting one of said points to the d-c connection at said other end of said at least one conductor, a monitoring resistance selected for value substantially corresponding to the desired safety-limit value of said ground conductor and in substantially the same proportional relationship therewith as the values of said fixed-resistance members bear to each other, said monitoring resistance connecting said other end of said ground conductor to said one point, thereby defining second conjugate members connected between said conjugate arms at the respective member-to-member connections thereof; and circuit-breaker trip means operative upon detection by said last-defined means of a predetermined voltage level.
 19. Transmission-line means according to claim 16, in which a system ground to earth is provided at said one end of said ground conductor and in which said other end includes a local-grounding frame member, and means including normally closed contacts at said other end and connecting the corresponding end of said ground conductor to said frame member, the ground connection of said bridge elements at said other end being made to said frame member, whereby upon opening said contacts said voltage-responsive means and said monitoring resistance may serve to measure then-existing earth resistance to the system ground via such path or paths as may exist between said frame member and the local electrical relation thereto to earth.
 20. Transmission-line means having a source-connection end and a remote load-connection end and comprising a ground conductor and multiple further conductors, including power conductors; a first d-c conducting connection of at least one of said multiple conductors to said ground conductor at one of said ends, said at least one conductor including a d-c connection at said other end; a second d-c conducting connection of another of said multiple conductors to said ground conductor at said one end; circuit-breaker means for said power conductors at the source-connection end; Wheatstone-bridge elements at the other of said ends and utilizing, as one corner of the bridge, the said interconnection of multiple conductors and ground conductor at said one end, so that the ground conductor itself may be one of the arms of a completed bridge; said bridge elements at said other end comprising two serially connected resistance arms of ohmic value much higher than that for which the ground conductor is to be monitored, thereby defining a second bridge corner at the interconnection of said high-resistance arms, a low-resistance arm of ohmic value comparable to that for whicH the ground conductor is to be monitored, said low resistance arm being connected to the otherwise-free end of one of said high-resistance arms, thereby defining a third bridge corner at interconnection of high and low-resistance arms, and thereby also defining first and second otherwise-available free ends of a high-resistance arm and of the low resistance arm respectively; means including said at least one conductor and connecting one of said otherwise-available free ends to said first-defined bridge corner, and means connecting the other of said otherwise-available free ends to said other end of said ground conductor, thereby defining the fourth bridge corner, and thereby establishing the series-loop connection of all four bridge corners via said arms; a voltage-excitation source diagonally connecting a first two opposite bridge corners, and voltage-responsive means diagonally connecting the remaining two opposite bridge corners, said source and voltage-responsive means being located at said other end, and said another conductor being the means of completing the requisite one of said diagonal connections to said first-defined corner; and means responsive to attainment of a predetermined response of said voltage-responsive means for tripping said circuit-breaker means.
 21. Transmission-line means according to claim 20, in which the high-resistance arms are of substantially matched ohmic value, and in which the ohmic value of the low-resistance arm is to a predetermined safety-monitoring level of ground-conductor resistance, said voltage-responsive means being polarity sensitive and operative to actuate said last-defined means upon detection of a polarity reversal of voltage. 